1. Field of the Invention
The present invention relates to a surface coated cutting tool having excellent wear resistance and chipping resistance, the cutting tool including a cemented carbide substrate whose surface is coated with a composite hard layer formed by means of chemical vapor deposition, the composite hard layer consisting substantially of an inner layer including one or more layers of titanium carbide, titanium nitride, titanium carbonitride, titanium carboxide, and titanium oxicarbonitride (hereinafter referred to generally as titanium compound layers), and an outer layer including at least one alumina layer.
2. Description of the Related Art
For the purpose of continuous and interrupted cutting of steel, it has been well known to use a surface coated cutting tool including a cemented carbide substrate whose surface is coated with a composite hard layer formed by means of chemical vapor deposition, the composite hard layer consisting substantially of an inner layer including a titanium compound layer and an outer layer including at least one alumina layer.
The alumina layer is chemically stable and excellent in face wear resistance, but is poor in adhesion property to the substrate and toughness. Thus, the surface of the cemented carbide substrate is coated first with a titanium compound layer formed by means of chemical vapor deposition, then an alumina layer is formed on the titanium compound layer by means of chemical vapor deposition, thereby giving wear resistance and toughness to the formed composite hard layer to improve chipping resistance.
As a method for forming the alumina layer of the composite hard layer by means of chemical vapor deposition, there is known a method for speeding up a growth rate of the alumina layer by use of reaction gas obtained by adding 0.01 to 1.0 vol. % of hydrogen sulfide gas to conventional reaction gas.
The use of this method shortens a time during which the inner layer made of titanium compound and the cemented carbide substrate are held at high temperature, thereby making the structure of the inner layer and the substrate unliable to change. Thus it is said that a surface coated cutting tool having a better performance than the conventional ones is obtainable according to this method (refer to, for example, Japanese Examined Patent Publication 62-3234).
The alumina layer obtained by use of reaction gas containing 0.01 to 1.0 vol. % of hydrogen sulfide gas is mainly an .alpha.-type alumina layer. It is also known that a K-type alumina layer is obtainable by changing the volume of carbonic acid gas in the reaction gas.
In recent years, there has been an increasing demand for a labor-saving and shorter cutting operation. This demand requires strongly a high speed continuous and interrupted cutting operation such as high speed feeding and high speed cutting under more severe conditions. During the high speed cutting operation, the temperature of a cutting edge rises above 1000.degree. C. and steel chips of exceedingly high temperature pass along a face of a cutting tool, which accelerates wear of the face. Thus, the cutting tool is chipped or damaged at a relatively early stage. Under these severe conditions, the cutting tools coated with the composite hard layer including the conventional alumina layer have had a relatively short lifetime of use because the coated layer has insufficient wear resistance and chipping resistance.